Semiconductor chips are conventionally placed onto chip carriers which are affixed to a printed wiring board for insertion into an electronic device. Design specifications require that the chip carriers be able to resist a series of temperature increases and decreases without failing. Temperature variations tend to cause failure in the solder joints where the chip carriers are connected to the printed wiring boards. A typical failure is a crack in the solder joint at the corners of the chip carrier, which cause a loss of electrical continuity. The cracks occur due to the different thermal expansion rates of the ceramic material from which the chip carrier is made and the material from which the wiring board is made, which may comprise for example, an epoxy/glass laminate. A typical ceramic chip carrier has an expansion rate in the range of 7 parts per million per degree Centigrade (ppm/.degree. C.) versus 15 ppm/.degree. C. of an epoxy/glass laminate printed wiring board.
One attempt to prevent failures in the solder joints has been to provide a better match between the temperature expansion rates of the chip carrier material and the printed wiring board material. Different kinds of chip carriers may be utilized in an attempt to better match the expansion rates; however, the printed wiring board material typically is not a viable option, especially when dealing with rigid military specifications. It has thus not been heretofore possible to find useable materials having thermal expansion rates that match closely enough to eliminate cracks in the solder joints.
Another attempted solution has been to apply more solder to the joint. This method overfills the joint with solder to form a convex surface. By adding more solder, a crack must travel further before total failure occurs. This method is merely a stopgap and is not a solution, as more solder has produced only limited success while adding cost and taking up valuable space.
Another attempt to resolve the problem has been to use a wire lead that can be soldered to the wiring board and brazed to the chip carrier. This method is expensive, takes up valuable space and still runs a high risk of failure. Thus, a need exists for an improved contact joint between a chip carrier and a printed wiring board that will reduce the occurrences of cracks in the solder due to temperature variations.